The employment of solutions or dispersions of organic polymers dissolved or dispersed in volatile organic liquids to formulate organic polymer coating compositions and their subsequent application onto various substrates as coatings thereon requires the handling and evaporation of large quantities of organic solvents. Because of the undesirable ecological and environmental problems and problems associated with the exposure of workers in the coatings industry to the organic solvents, alternate coating methods have become a necessity. Consequently there has been a shift to coating techniques where water is substituted for much of the volatile organic solvent or diluent in the coating compositions. These are often referred to as "water borne" coating compositions even where water is not the sole dispersing or dissolving vehicle for the organic polymer.
The rise in the use of water borne coatings has introduced problems peculiar to systems containing a mixture of water and organic cosolvent or dispersant. These problems arise form the fact that the evaporation of water is dependent upon the ambient humidity conditions, and the relative rates of evaporation of the organic solvent vis a vis water.
Polymers which were developed for water borne coatings systems were often based on the chemistry of the polymers used in existing solvent systems. Thus alkyds, epoxy esters, and oil modified polyurethanes originally developed for nonaqueous formulations were modified for aqueous formulations by the incorporation of acid moieties.
One of the earliest techniques for the incorporation of acid groups was to maleinize a long oil alkyd or epoxy ester. In this process the alkyd or polyester is reacted at 200.degree.-260.degree. C. for 30-60 minutes with maleic anhydride in the presence of an excess of drying acid. The excess drying acid serves as a reactive chain transfer agent to prevent premature gelation. Accelerators, commonly iodine or sulfur dioxide, were often employed in this thermal polymerization process. The maleic anhydride moiety (now a succinate) is hydrolyzed and neutralized with volatile and/or fixed bases to render the polymer soluble in the water/cosolvent mixture. Enough unsaturation is left in the resin for siccative cross-linking of the final film.
Another early technique was to use dimethylol propionic acid (derived by condensation of propanal with formaldehyde followed by oxidation). This unique acid-diol along with the monoglyceride is reacted with phthalic anhydride or isophthalic acid to form a polyester/alkyd having free acid groups. The carboxyl moiety of the dimethylol propionic acid is sterically hindered and does not esterify during the polymerization but reacts subsequently with the neutralizing base thus solubilizing the polymer.
Oil modified urethanes are produced by substitution of a diisocyanate for the phthalic anhydride. The urethane reaction is carried out at moderate temperatures (50.degree.-100.degree. C.). These types of polymers form the basis of water born coating materials used today in low cost product and consumer finishes.
The next innovation was the development of resins that could be cross-linked by reaction with aminoplasts. These resins contained available hydroxyl functionality as well as an ionizable moiety. The acid containing polymers were used early in electro-coating, and were synthesized by partially esterifying an epoxy resin with a drying fatty acid followed by reaction with trimellitic anhydride. The carboxyl groups incorporated were neutralized in the usual manner. Esterification of the partial epoxy ester with para-aminobenzoic acid in place of trimellitic anhydride yielded early cationic polymers. The cationic polymers are, commonly neutralized with acetic, lactic and other volatile organic acids to attain solubility.
Today the resins are highly specialized for their intended applications. Techniques for the incorporation of the ionic moieties include graft polymerization of carboxylic vinyl monomers to the base epoxy and polyester backbones as well as the development of functionalized telechelic polymers from readily available monomers.
As the coatings industry moves, because of environmental and health regulations, from the more conventional solvent coatings to water borne and high solids systems, there has developed a need for new water borne reactive diluents and reactive cosolvents to serve this emerging technology. At the present time many of these end use needs are being addressed by solvents and reactive materials already in commerce. However, none of these materials ere designed for these applications; rather the industry adapted what was available. Today the state of the art has advanced to the point where the real needs are apparent and further improvement in the coating systems requires improved reactive co-solvents and reactive diluents as well as polymers.
For example many of the prior art resin solvents used in water borne coating systems led to degraded polymer properties in the final coatings. Water borne coatings as used herein are coatings in which the principal application medium is water. They may be of two types: (1) dispersions in which organic materials are added at low levels as filming acids, antifreeze additives, defoamers, etc.; (2) solutions in which part of the application medium is an organic solvent. Performance requirements demand that coatings resist the adverse effects of water in the environment and yet in water born solution coatings technology they are applied from a solution which contains a large fraction of water. This requires that the coatings are not truly water soluble; rather, a number of techniques are employed to maintain solubility throughout application and film formation. The techniques commonly employed are:
(a) Ionic groups are incorporated into the polymer, PA1 (b) A means of crosslinking the film after application is employed, PA1 (c) A cosolvent is used to maintain solubility of the polymer throughout the film forming process.
In the case of solution coatings, it is imperative that during the drying stage, after a particular substrate has been covered with a layer of coating composition that a single phase be maintained until the water and cosolvent components have evaporated away leaving the now insoluble organic polymer deposit. It is also necessary that the cosolvent be miscible with the water and that the organic polymer coatings be soluble in the cosolvent.
The relative volatility of the cosolvent with respect to water involves vapor pressure, molecular weight and the relative humidity during the drying operation. Under high humidity conditions the rate of water evaporation becomes very slow while the evaporation of the volatile organic co-solvent remains relatively constant. Consequently under conditions of high humidity, imperfections develop during film formation which are detrimental to the overall performance of the coating. These imperfections which form under high humidity conditions are related to the limited solubility of the film forming polymers in the resulting water rich composition. The problem becomes understandable when it is cast in terms of the phase chemistry of a water borne coating.